natural logarithm for the extended reals and some simple lemmas

[jmmarulang]

Motivation for this change

the natural logarithm for the extended reals is needed for extending hoelder into the extended reals.

Checklist

• [X] added corresponding entries in CHANGELOG_UNRELEASED.md

• [X] added corresponding documentation in the headers

Reference: How to document

Merge policy

As a rule of thumb:

• PRs with several commits that make sense individually and that all compile are preferentially merged into master.
• PRs with disorganized commits are very likely to be squash-rebased.

Reminder to reviewers

• Read this Checklist
• Put a milestone if possible
• Check labels

[affeldt-aist]

@jmmarulang This is your master that you are PRing, is that ok?

[affeldt-aist]

@jmmarulang This is your master that you are PRing, is that ok?

I will open a new PR to avoid any problem.

[affeldt-aist]

@jmmarulang since PR #1649 has been merged, is it ok to close this draft PR?

[jmmarulang]

yes thank you!